There are many applications where a need exists to generate an electromagnetic field, for example a magnetic field, which is tailored to have a strong near field and a weak, rapidly vanishing far field. Such applications include scanning electron microscopes, various electron deflectors, magnetic bearing devices and magnetic positioners/electric motors; however, the primary current application for such electromagnetic fields is in electron beam lithography. According to the Semiconductor Industry Association in its National Technology Roadmap for Semiconductors, the required image placement accuracy for electron beam lithography production machines is projected to be 14 nm in the year 2003 and only 6 nm in the year 2012. As a result, electron beam stages will have to satisfy demanding requirements imposed on their placement accuracy, stray magnetic fields, thermal stability, and structural performance. Further, to reduce the time required to produce a chip, particularly a large chip which may include an array of 1,000.times.1,000 individual circuits, it is desirable that multiple electron guns be operating in parallel. However, with existing technology, there may be interaction between adjacent electron guns, severely limiting the number of guns which may be used in parallel. To complicate matters, the stages must also meet strict vacuum compatibility requirements since electron beam systems typically operate at high vacuum (&lt;10.sup.-6 Torr).
Recently developed motors with precise motion control and a single, magnetically levitated moving part possess several desirable features for application in state of the art lithography, as described in U.S. Pat. Nos. 5,699,621 and 5,196,745. A magnetic bearing stage can be mechanically simple, yet highly functional, providing movement with six degrees of freedom for a single moving part. Since the stage does not require precision bearing surfaces, which are susceptible to friction and backlash, positioning performance can be extended to the limits of control and metrology, and fabrication costs can be reduced. Also, disturbances that induce substrate vibrations can be controlled. For instance, use of a single moving part, as opposed to a plurality of moving parts, typically results in a structure with a higher natural frequency. Therefore, the stage is less sensitive to vibration and is more stable. Further, the stage has an increased travel speed and permits greater machine throughput. Since no lubrication is required and the suspended stage does not generate wear particles, a magnetic bearing stage is highly suited to both cleanroom and a vacuum environments. Magnetic bearing stages are highly reliable mechanical systems, an important system-level consideration for maintaining machine up-time. Additionally, stringent thermal stability requirements for the substrate can be accommodated since there is a gap between the stage and power dissipating components providing a large thermal resistance.
FIGS. 1(a) and (b) show the basic structure of such a prior art synchronous motor 10 with a magnetic bearing stage. The motor includes a Halbach magnet array 12 (see K. Halbach, Nuclear Instruments and Methods, vol. 169, 1980, p. 1-10 for an introduction to Halbach arrays) and a stator 14. Array 12 is a block array with four permanent magnets per period as designated by a magnetization rotation period. Stator 14 has six current phases, three which are designated A, B and C and three which flow in the reverse direction, respectively, and are designated A', B' and C'. One with ordinary skill in the art would recognize that other phases and/or magnet periods and/or magnets per period can be used and that electromagnets rather than permanent magnets can be used in the Halbach array. Strong forces are generated in the translation and suspension direction. Motion can be generated in the suspension and translation direction by varying the magnitude of the currents, and translations of array 12 relative to stator 14 can be generated by commuting the current phases in a sinusoidal manner. A small gap between the array 12 and the stator 14 serves to mechanically and thermally decouple these components.
Due to the aforementioned advantages, magnetic bearing stages provide a promising new alternative to stage technology currently being used in electron beam machines. However, charged particle motion is very sensitive to magnetic fields, and therefore, small uncontrolled changes in the magnetic fields experienced by the beam can cause significant and adverse deflections of the beam. For example, the magnitude of fringing fields must be less than 2 mGauss in order to keep Lorentz force deflections below 1 nm in a typical electron lithography beam. This poses a significant constraint on the magnetic fields that can be tolerated from the stage.
The magnetic field generated by a Halbach array of infinite extent decays exponentially with perpendicular distance from the surface due to efficient cancellation of the magnetic far-fields. In practice, however, Halbach arrays are always finite in size and, in fact, are preferably small in order to have a small footprint and mass. The magnetic field of an array with a finite extent decays more slowly than exponentially due to a lack of cancellation of the magnetic far-fields generated at ends of the array. The most rapidly vanishing magnetic field component generated by a Halbach array described in the prior art is quadrupole-like and decays as 1/r.sup.4, where r is distance from a point of observation to the array and is assumed to be large compared to the size of the array.
In order to incorporate a magnetic bearing stage into an electron beam machine, electromagnetic fields generated by the stator must also be small. Again, because of the finite size of practical stators, there is poor cancellation of the fields originating from ends of prior art stators. Generally, stators include current bearing coils, each of which acts as a magnetic dipole. Therefore, the magnetic fields of prior art stators appear dipole-like (1/r.sup.3 drop off) or, at best, quadrupole-like (1/r.sup.4 drop off) if the dipole-like contributions from the ends happen to cancel.
What has been said above for magnetic arrays and coils for use in drive motors for electron beam devices applies equally for other applications of magnetic arrays, coils or other electromagnetic field sources where strong near field and weak, rapidly vanishing far field requirements exist, and current technology can not provide better than quadrupole-like (1/r.sup.4) drop-off. For example, electromagnets with rapidly decaying far fields would permit more dense packaging of electron guns without interference between adjacent guns, thus facilitating increased parallel generation on a chip, perhaps even permitting all circuits on a given chip to be simultaneously generated. Furthermore, current technologies provide no method to attenuate the fields of underlying quadrupoles.
A need, therefore, exists for a technique for attenuating far-fields generated by magnetic bearing stages, electron guns, or other magnetic or electromagnetic sources, in order to permit their use in magnetically/electromagnetically sensitive applications. Such applications include, but are not limited to, electron beam lithography. Magnetic bearing stages may also be used in other charged particle machines such as ion beam machines to hold samples for ion implantation or for characterization by secondary ion mass spectroscopy for instance. One with ordinary skill in the art will easily recognize the potential use of such a stage in other magnetically sensitive applications.
One potential method for attenuating magnetic fields is the use of shielding. However, incorporating magnetically permeable material onto a moving stage would result in undesirable uncontrolled distortions of stray fields which are likely to be present. For example, uncontrolled deflections of fields generated by magnetic lenses in the charged particle machines would correspondingly affect the charged particle motion. Alternatively, a stationary shield may be used between magnetic components and the beam. Holes in the stationary shield would allow structures supporting substrates, for instance, that are to be patterned by an electron beam on one side of the shield, to be attached to the moving stage located on the other side of the shield. The holes must be spaced far from the magnetic components in order to permit the shield to be effective. However, such a configuration also has several disadvantages including a large size which tends to produce a large footprint and poor dynamic properties.
Since space is expensive and is always at a premium in clean-room environments used for electron beam lithography semiconductor manufacture, any technique for reducing the size/footprint of various components of such a system is also desirable.